This invention relates to time measuring systems, and more particularly to time measuring systems which are suitable for automatic focus cameras or other systems requiring a large dynamic range for time measurement.
In many automatic focus cameras, distance is measured by triangulation. To use that principle, two optical systems in the cameras form an image of an object on two light receiving element (or photo-sensor) arrays. The distance between the object and the camera is then determined by correlating the differences between the two images.
One method of converting an optical representation of the object's image into an electrical signal is shown in FIGS. 1-3. FIG. 1 is a circuit diagram of a photosensor in a photosensor array. FIG. 2 is a timing chart showing the operation of the photosensor in FIG. 1. FIG. 3 is a block diagram of a distance measuring device using photosensors such as those shown in FIG. 1.
In FIG. 1, a photosensor 10 includes a photodiode 11, a coupling capacitance (or a capacitor) 12, an inverter 13, and a reset transistor 14. Photosensor 10 is started by a RESET signal (FIG. 2(a)) which renders transistor 14 conductive causing capacitor 12 to discharge. At the end of the RESET signal, capacitor 12 begins integrating the current through photodiode 11 and produces a voltage V as shown in FIG. 2(b). When the voltage V reaches a predetermined value, shown in FIG. 2(b) as V.sub.th, the threshold voltage of inverter 13 in FIG. 1, the inverter 13 changes state and the output S of inverter 13 drops to a low ("L") level, as shown in FIG. 2(c).
The circuit in FIG. 1 thereby converts received light intensity into a binary signal S according to the time required for the integration voltage to reach the threshold voltage V.sub.th. Photosensor 10 in FIG. 1 is also referred to as a "conversion element" or "conversion sensor," and the integration time of capacitor 12 (time T in FIG. 2(c)) is also referred to as a "conversion element time" or "sensor response time."
One disadvantage of such circuit is that the change in V with time, dV/dt, depends on the quantity of light received. Therefore, different sensors sometimes provide different outputs for the same light. This disadvantage can be exacerbated in a distance measuring device which includes several photosensors 31 of the type shown in FIG. 1.
Such a device is shown in FIG. 3. The outputs Si.sub.1, Si.sub.2. . . S.sub.in of photosensors 31 are each inputs to a different one of AND gates 33a, 33b . . . 33n (collectively denoted 33). A clock pulse .phi..sub.1 is a second input to each AND gate 33. The outputs of AND gates 33 are each an input to a different one of counters 44a, 44b . . . 44n (collectively denoted 44) which digitally quantize the photosensors' response times.
If the measuring device of FIG. 3 uses a short clock pulse period (high frequency) to ensure accurate measurement of short time periods, then the device must include a great deal of additional circuitry, such as counters, to measure long time periods also.
Accordingly, an object of this invention is to provide a time-measuring system which can accurately measure a wide range of time periods without needlessly increasing the amount or complexity of associated circuitry.